The present invention is related to an improved method of packaging a solid electrolyte capacitor and an improved capacitor formed thereby. More specifically, the present invention is related to improving the volumetric efficiency of a capacitor by an improved method of positive and negative termination in a capacitor and an improved capacitor formed thereby.
The construction and manufacture of solid electrolyte capacitors is well documented. In the construction of a solid electrolytic capacitor a valve metal serves as the anode. The anode body can be either a porous pellet, formed by pressing and sintering a high purity powder, or a foil which is etched to provide an increased anode surface area. An oxide of the valve metal can be electrolytically formed to cover all surfaces of the anode and to serve as the dielectric of the capacitor. A solid cathode electrolyte is formed on the dielectric and is typically chosen from a very limited class of materials including manganese dioxide or electrically conductive organic materials such as 7,7′,8,8′-tetracyanoquinonedimethane (TCNQ) complex salt, or intrinsically conductive polymers, such as polyaniline, polypyrrole, polyethylenedioxythiophene and their derivatives. An important feature of the solid cathode electrolyte is that it can be made more resistive by exposure to high temperatures. This feature allows the capacitor to heal leakage sites by Joule heating. In addition to the solid electrolyte the cathode of a solid electrolyte capacitor typically comprises several layers which are external to the anode body. In the case of surface mount constructions these layers typically include: a carbon layer; a layer containing a highly conductive metal, typically silver, bound in a polymer or resin matrix; a conductive adhesive layer such as silver filled adhesive; and a highly conductive metal lead frame. The various layers connect the solid electrolyte to the outside circuit and also serve to protect the dielectric from thermo-mechanical damage that may occur during subsequent processing, board mounting, or customer use.
Continued efforts are directed towards increasing the volumetric efficiency of solid electrolytic capacitors. The volumetric efficiency of an electrolytic capacitor is typically defined as the ratio of the active capacitor volume to the volume of the entire molded capacitor package. The anode leadwire typically extends axially from the anode to a lead frame and ultimately to an external termination. This assembly occupies a significant amount of space inside the capacitor package.
As the volumetric efficiency of capacitors has increased the parasitic resistance has also increased. This has led to a particular conundrum in the industry with much effort being spent attempting to decrease the size of a capacitor without increasing parasitic resistance. These goals have previously been considered contradictory.
U.S. Pat. No. 7,161,797 describes a method for increasing volumetric efficiency. This method requires careful cutting of an encapsulated pellet followed by formation of an electrode on the exterior of the encapsulated body. While advantageous for volumetric efficiency it is desirable to have more contact between the anode wire and conductor than that afforded by the end of the anode wire. An exterior termination is undesirable in many applications, particularly, when many components are closely packed as would be the case when volumetric efficiency is a concern.
The present application provides a capacitor, and method of making the capacitor, wherein the volumetric efficiency is improved without significant negative impact to parasitic resistance.